1. Field of the Invention
The present invention relates to an image forming apparatus that can perform multiple-exposure processing for forming an electrostatic latent image on an image carrier surface. More specifically, the present invention relates to an intensity control for exposure spots placed one upon another at the same exposure point.
2. Description of the Related Art
A conventional image forming apparatus can form an electrostatic latent image on a charged image carrier surface by performing scanning exposure using a plurality of laser beams and can develop a toner image with charged toner particles supplied onto the electrostatic latent image formed on the carrier surface. For example, a conventional apparatus can use a plurality of laser beams to form an image at the same position on an image carrier.
As discussed in Japanese Patent Application Laid-Open No. 11-326798, using semiconductor lasers differentiated in the size of optical spot is useful to smoothly reproduce a peripheral region of a latent image and form a combined toner image in a desired manner. As discussed in Japanese Patent Application Laid-Open No. 09-169136, a first beam having a beam diameter of 30 μm and a second beam having a beam diameter of 60 μm can be combined with each other while setting the ratio of 5:3 in the light quantity of light emission.
In general, the distribution of exposure amount at an exposure spot formed on an image carrier surface is a Gaussian distribution having a peak value at the center and peripheral values gradually decreasing as the distance from the center increases. Therefore, the electrostatic latent image of an exposure spot formed when electric charge is discharged with a current value corresponding to the exposure amount has an electric potential distribution (distribution of electrostatic charge amount) having a peak value at the center and peripheral values gradually approaching to a charging potential, which reflects the distribution of exposure amount.
The development of a toner image is dependent on a relationship between the potential distribution of an electrostatic latent image at the exposure spot and a development voltage set for a developing device. The potential distribution of an electrostatic latent image has an effect on the size or the density of a developed toner image (refer to FIGS. 9A and 9B). The size or the density of a toner image, even if the electrostatic latent image is the same, changes if any variation occurs in the development voltage or in the electrostatic charge amount of a developer.
More specifically, the size of a toner image if formed by developing an electrostatic latent image at the exposure spot is variable according to the potential distribution of an electrostatic latent image, the development voltage, or the electrostatic charge amount of a developer. If any change occurs in the size of a toner image resulting from the exposure spot, the line width of the toner image obtainable by scanning the exposure spot changes correspondingly. The density of an image formed by arraying development lines is also changed.
As described below, if the image carrier goes into a late stage of its life cycle, an electrostatic latent image having the same potential distribution cannot be reproduced even if the exposure is performed under the same conditions. The line width and the image density cannot be reproduced correctly for the same toner image.
Furthermore, depending on usage conditions of a developing device (e.g., deterioration in toner particles as a recording material) or if any change occurs in environmental conditions, the line width and the image density cannot be reproduced accurately for the same toner image to be produced from the same electrostatic latent image.
Furthermore, as discussed in Japanese Patent Application Laid-Open No. 2003-323009, the exposure of a pixel can be realized by a plurality of lasers. According to the configuration discussed in Japanese Patent Application Laid-Open No. 2003-323009, in order to accurately combine toner images, toner images formed by respective light beams are equalized. It is also required to equalize a combined toner image formed by placing light beams one upon another with each toner image formed by each light beam. Hence, to maintain the latent image potential at an appropriate level, the output power adjustment for each laser is performed every time the number of sheets reaches a predetermined value.
However, according to the configuration discussed in Japanese Patent Application Laid-Open No. 2003-323009, each laser has the same spot size and, if the laser power is adjusted to attain a target density level when the potential distribution changes due to deterioration, the line width of a latent image (potential distribution) changes correspondingly and the line width of a toner image is also changed. On the contrary, if the laser power is adjusted to maintain the same line width, the target density cannot be maintained.